A honeycomb structure is built into an exhaust system of a diesel engine and the like to be used as a trapping filter for exhaust gas, for example, as a diesel particulate filter (DPF) to trap and remove particulate matter (particulates) contained in an exhaust gas from the diesel engine and the like.
Such a honeycomb structure has a structure in which a plurality of cells functioning as flow channels of fluid partitioned and formed by porous partition wall made of, for example, silicon carbide (SiC) are disposed in parallel with one another in the direction to the central axis. Further, ends of adjacent cells are alternately plugged (in a checkered pattern). That is, in one cell, one end is open, and the other end is plugged, and in another cell adjacent to this cell, one end is plugged, and the other end is open.
Because of such a structure, the exhaust gas allowed to flow into predetermined cells (inflow cells) from the end portions on one side is allowed to flow out through the adjacent cells (outflow cells) by allowing the exhaust gas to pass through the porous partition walls, and when passing the exhaust gas through the porous walls, the particulate matter (particulates) in the exhaust gas is trapped by the partition walls, so that the purification of the exhaust gas can be performed.
To continuously use such a honeycomb structure (filter) over a long period of time, a filter must be regenerated. That is, to remove an increase in pressure loss by the particulates accumulated with time inside the filter, the particulates accumulated inside the filer with time must be burnt and removed. At this regeneration time of the filter, there has been a problem that a large thermal stress is generated to cause a defect such as a crack and breakage. Complying with the request for improvement of thermal shock resistance against such a thermal stress, there has been proposed a honeycomb structure of a split structure allowed to have functions to disperse and relax the thermal stress by integrally bonding a plurality of honeycomb segments by a bonding material layer, thereby enabling it to improve the thermal shock resistance to some extent. The honeycomb structure of such a split structure is formed such that each has a shape to configure a part of the entire structure, and, at the same time, a plurality of honeycomb segments having a shape to configure the entire structure by being fitted in the direction perpendicular to the central axis are integrally bonded by the bonding material layer and are formed so as to become a predetermined shape such as a circle and the like in the entire cross-sectional shape cut along a plane perpendicular to the central axis, and after that, its outer peripheral surface is covered by a coating member.
However, in recent years, the filter has an increasing demand for a large size, and the thermal stress generated at the generation time is also increasing, and therefore, to inhibit the above described defect, the improvement of thermal shock resistance as a structure has comes to be strongly desired. Above all, the bonding material layer for integrally bonding a plurality of honeycomb segments is desired to realize a honeycomb structure excellent in thermal shock resistance by realizing an excellent stress relaxation function and bonding strength.
Addressing such a problem, a ceramic structure (honeycomb structure) has been disclosed (see Patent Document 1) in which, by adding inorganic fibers and organic binders to a sealing member (bonding material layer), generation of migration in the process of drying and hardening is suppressed, thereby controlling a development of the above described defect and aiming at the improvement in durability.
Further, there has been disclosed a honeycomb structure (use of a low Young's Modulus bonding material) satisfying either one of the conditions that the Young's Modulus of the bonding material layer between the honeycomb segments is made not more than 20% of the honeycomb segment material or that the material strength of the bonding material layer is made smaller than the material strength of the honeycomb segments (see Patent Document 2).
However, in the sealing member (bonding material layer) used for the ceramic structure (honeycomb structure) disclosed in the Patent Document 1, there has been a problem that securement of a bonding strength between the segment and bonding material layer boundary surface and securement of a stress relaxation function of the bonding material layer itself are hard to be compatible in the uniform system realized by the mutual inter-twisting of the component inorganic fibers and organic binders.
Further, though the low Young's Modulus of the bonding material forming the bonding material layer disclosed in Patent Document 2 is effective in that the thermal stress and thermal deformation generated between the honeycomb segments are relaxed, when the bonding material has high porosity due to the low Young' Modulus, there has been a problem that a bonding strength between the honeycomb segments becomes insufficient, and no sound honeycomb segment bonded body can be obtained.
Patent Document 1: JP-B-3121497
Patent Document 2: JP-A-2001-190916